Speckle is induced by light interference emerging from a screen. As shown in FIG. 1, the presence of speckle results in a granular pattern and can mask image formation.
The origin of this granular pattern is the extremely rough screen surface on the scale of the incident (e.g. optical) wavelength. Under illumination of coherent radiation, such as lasers, the wave reflected from such a surface consists of contributions from many independent scattering areas. Propagation of this reflected light to a distant observation point results in the addition of these various scattered components with relative delays, which may vary from several to many wavelengths. Interference of these de-phased but coherent wavelets results in the granular pattern known as speckle. Specifically, the constructively interfered wavelets lead to bright spots; the destructively interfered wavelets lead to the dark spots; the partially constructively interfered and partially destructively interfered wavelets lead to the intermediate or grey spots.
In order to improve the image quality in a laser display system, or any other system that may encounter speckle, it is necessary to reduce the speckle effect. The amount of speckle may be measured or calculated by Contrast Ratio (CR): CR=s/I, where I is the mean intensity and s is the root mean square intensity fluctuation. Speckle can be reduced by creating a number of independent speckle patterns and averaging them out within an integration time of a detector viewing the image, such as human eyes. Speckle contrast may be reduced to 1/√{square root over (N)} by superimposing N independent speckle patterns within the integration time. Speckle reduction may be achieved by using a vibrating screen. However, this technique may not be very practical due to the required high-power actuator. Speckle can also be reduced at the illumination source, by using multiple sources such as laser arrays or wide-band laser, for example. However, the special designed laser arrays may increase costs accordingly.
US2004/0008399 describes a method to suppress the speckle by creating N independent speckle patterns using a Hadamard Matrix (HM) phase mask as the diffuser, which is located on an intermediate image plane. A large HM phase mask is etched on a fused silica plate and then mechanically moved to create different speckle patterns at the screen when a laser beam passes through. However, the mechanical components required in this approach limit the technique and are impractical, especially when a high order HM is introduced to achieve a higher degree of speckle reduction. Using mechanical vibration, an actuator with a larger vibrating distance and a high working frequency is required.
Therefore, there is required a speckle reduction technique that overcomes these problems.